SMS is considered as really obsolete in Europe, even older people use WhatsApp. I think I have not sent a single SMS in the last 2 years. In Asia there are other services like WeChat (China) or Zalo (Vietnam) that are really popular, it really depends on the country. Sending an SMS feels like sendig a fax.
Regarding iMessage, I guess it's the same as BBM in the late 2000's, if you don't have it people might consider you are not cool enough to talk to them.
In Europe: WhatsApp, no one uses SMS anymore because WhatsApp started getting popular when people still had to pay for each SMS sent.
I currently live in South America, same story: Nobody uses iMessage, everybody is on WhatsApp. Even businesses prefer WhatsApp over phone calls.
The biggest difference between Whatsapp users in Europe and South America that I observed has to be the crazy amount of people using voice messages to communicate. Why don't you call and speed up the conversation by 50%?
That's not true, it may be true for some parts of Europe, but other countries had free or extremely cheap SMS before WhatsApp even existed.
Depending on where you are in Europe SMS may still be vastly more popular than WhatsApp. It also depends on the audience. Most of my family still use SMS. Technically it's iMessage but they don't know the difference, and they certainly don't care how messages are transferred. The build in messaging app is what is used.
Personally I've seen WhatApp installed only by people who have a large number of friends in eastern European countries.
Edit: Just to be clear, Denmark have had the one of the most competitive markets for cellular in the world for the past 20 years. Prices have been driven ever downwards for the last 15 year. So calls and sms have almost always been cheap.
It is asynchronous communication, like emails, the recipient can listen to it when they are available, but you don't need both to be available in the same time.
In ex-USSR SMS are dead, except on old "feature" phones. While in capitals a lot of people have iPhones it's share is not more than 50% and so iMessage never really picked up. Currently messaging it split between Viber (mostly older people) and Telegram (mostly younger people), plus some FB Messenger with FB friends. Whatsapp lost out market when it cost 1$ to install (yes, for many it was a decisive factor). iPhone users just install these apps too.
When traveling in Europe I usually see Whatsapp as a contact method.
I use SMSes for communication with people I don't know. For example, when buying/selling used stuff. For known people it's mostly FB messenger and WhatsApp, but sometimes also Skype and Viber. And then Slack for job related communication.
Different people use different channels, so I use different apps to be able to talk to anyone I need to.
WhatsApp, mostly, but also FB Messenger and Telegram depending on country. I just checked my phone and I have received just one SMS from a human this year.
Not Europe, but here in NZ I find most Apple owners use iMessenger. Android owners seem to prefer WhatsApp. However I'm finding more and more people using Facebook Messenger. As an Android owner I only really SMS with my husband's iPhone when I need messages to get through instantly but otherwise I just use whatever app the other person has. SMS is mostly for work contacts now days.
In The Netherlands (and most of Europe probably) for normies Whatsapp is definitely number 1, although most people will simultaneously use Facebook/messenger. Within businesses Microsoft Teams is starting to get popular, whereas Zoom seems to be more of US thing.
Wealthier "normie Apple families" use IMessage, but they'll always have a Whatsapp account on the side to talk to "the others".
Dutch nerds/early adopters will sometimes use Signal, but it's definitely not a thing most people know about. Telegram is even rarer in my experience.
French government uses Matrix/Riot.im
There are exceptions to the above though. I don't use Whatsapp or IMessage at all and never used it for more than a few days total in my entire life. My main messaging app is Signal and I'm totally fine. Got about 50 contacts on there and that's more then enough :D
Nightmare scenario for me: Google successfully integrates whatever there newest messaging brand is into the Android messages SMS client (now rebranded to Google messages) and all Android users switch to that because it's default. Result: Google messages becomes IMessage for Android, including the vendor lock-in with added tracking as a bonus. I feel this is happening.
What do people use in Europe?